This invention relates to an antenna, and in particular a dual resonance antenna.
With the increasing demand for mobile communications different cellular standards have been developed, many of which operate at different frequencies. For example, the global system for mobile communication (GSM) standard defines the primary frequency band for GSM as being from 890 MHz to 960 MHz, while the digital cellular system (DCS) standard defines the primary frequency band for DCS as being from 1710 MHz to 1880 MHz.
The different cellular systems can operate in isolation or together. To maximise the use of these different cellular systems and increase the use and mobility of mobile communication devices it is desirable for mobile communication devices to be able to roam between the different cellular systems.
To allow a mobile communication device to roam between cellular systems having different operating frequencies the communication device will typically need a dual resonance antenna with one resonating element tuned to one cellular system and a second resonating element tuned to another cellular system. The dual resonance antenna, otherwise known as a dual band antenna, may be in the form of two physically separate antenna housings having separate resonating elements that are fed via the antenna feed. Alternatively, the antenna may have two resonating elements physically coupled in the same housing, with each element having a different resonant frequency.
However, as electronic and communications technologies have advanced, there has been a drive to increase the performance and decrease the size of consumer devices. In particular, in the field of mobile communications, there has been continual demand for increasingly smaller communications devices, such as telephones, computers and personal organisers, but without a decrease in performance. However, as electronic equipment has rapidly reduced in physical size due to the development of integrated circuits, the antenna for communication equipment still remains large compared with the equipment itself.
From the point of view of facilitating the operation of mobile communication devices low profile antennae suitable for mounting within a communication device have become increasingly popular. An example of such an antenna is a planar inverted antenna where coupling the resonating element to a ground plane to produce a planar inverted F antenna (PIFA) can halve the length of the resonating element.
A PIFA comprises a flat conductive sheet supported a height above a reference voltage plane such as a ground plane. The sheet is typically separated from the reference voltage plane by a dielectric, for example air. A corner of the sheet is coupled to the ground via a grounding stub, otherwise known as a shorting pin, and a feed is coupled to the flat sheet near the grounded corner for driving the antenna. The feed may comprise the inner conductor of a coaxial line. The outer conductor of the coaxial line terminates on and is coupled to the ground plane. The inner conductor extends through the ground plane, through the dielectric (if present) and to the radiating sheet. The PIFA forms a resonant circuit having a capacitance and inductance per unit length. The feed point is positioned on the sheet a distance from the shorting pin such that the impedance of the antenna at that point matches the output impedance of the feed line, which is typically 50 ohms. The main mode of resonance for the PIFA is between the short circuit and the open circuit edge. Thus the resonant frequency supported by the PIFA is dependent on the length of the sides of the sheet and to a lesser extent the distance and the thickness of the sheet.
However, a dual band PIFA antenna having two resonating elements still increases the size of the antenna thus compromising the ability of the antenna to be mounted within a communication device.
In accordance with an aspect of the present invention there is provided an antenna comprising an electrical reference plane; a planar conductive element, the electrical reference plane and planar conductive element being electrically coupled via a first coupling means to define a first antenna resonant frequency; and a second coupling means arranged to provide a high impedance path between the electrical reference plane and the planar conductive element at the first antenna resonant frequency and a lower impedance path between the electrical reference plane and planar conductive element at a second frequency to define a second antenna resonant frequency.
This provides the advantage of a dual band antenna having a smaller size than a conventional low profile dual resonance antenna.
The overall electrical length of the planar conductive element determines the antenna""s resonant frequency. When the planar conductive element, otherwise know as a resonator element, has a single coupling to the reference plane the electrical length, and hence resonance, is determined by the length and width of the resonator element with respect to the coupling. When the resonating element has a second coupling to the reference plane the electrical length is determined by the width of the element and the distance between the two coupling points. Thus a single resonator element can have a number of different electrical lengths depending on how the element is electrically coupled to the electrical reference plane.
Further, the first resonant frequency can be tuned by varying the length of the resonator element while the second resonant frequency can be tuned by altering the position of the coupling of the second coupling means to the resonator element. Thereby, the present invention provides the advantage of allowing the first and second resonant frequencies to be tuned substantially independently.
Generally the antenna includes a feed section comprising the first coupling means and a conducting element arranged parallel to each other with the conducting element being connected to a feed such that the first coupling means and the conducting element form a transmission line.
Since the feed section is arranged as a transmission line, energy is contained and guided between the conductors of the transmission line. This results in a low Q factor and hence a higher impedance bandwidth for the first resonant frequency compared with conventionally fed planar antennas. Thus, the bandwidth is increased considerably while retaining the efficiency, size and ease of manufacture of planar antennas.
Suitably, the second coupling means comprises a filter.
By using a filter which has a high impedance at the first resonant frequency and a low impedance at the second resonant frequency the planar conductive element can have two resonant frequencies simultaneously.
Preferably, the second coupling means comprises a switch movable between a first position for electrically isolating the electrical reference plane and planar conductive element and a second position for electrically coupling the electrical reference plane and planar conductive element.